1. Field of the Invention
The subject invention relates to orthopedic prosthetic components, and particularly to a knee joint prosthesis.
2. Description of the Related Art
A natural knee joint includes the distal end of the femur, the proximal end of the tibia and a meniscus bearing therebetween. The femur and the tibia are held in proper relationship to one another and to the bearing by a plurality of ligaments, including the posterior cruciate ligament, the anterior cruciate ligament and collateral ligaments. Flexion of the knee joint causes the tibia to rotate relative to the femur about an axis extending generally in a medial-to-lateral direction. Flexion also generates rotation of the tibia about its own axis.
Damage or disease can affect the ability of the natural knee to function properly. The damage or disease can deteriorate the bones, the articular cartilage, the ligaments or some combination thereof. A damaged or diseased natural knee can be replaced by a prosthetic knee joint. A prior art knee joint prosthesis includes a femoral component securely mounted to the distal end of a resected femur, a tibial component securely mounted to the proximal end of a resected tibia and a bearing disposed between the femoral and tibial components. The inferior face of the femoral component includes a pair of condyles. The condyles have a convexly arcuate shape, and the superior surface of the bearing has a pair of arcuate concave regions for articular bearing engagement with the condyles of the femoral component. The superior face of the tibial component may be substantially planar and is in bearing engagement with the inferior face of the bearing.
Currently available prosthetic knee joints take many different forms depending upon the preferences of the orthopedic surgeon, the condition of the natural knee and the health, age and mobility of the patient. Some prior art knee joint prostheses fixedly mount the inferior surface of the bearing to the superior surface of the tibial component. Other knee joint prostheses permit rotary movement and/or sliding movement between the bearing and the tibial component. Movement of the bearing against the tibial component achieves many functional advantages described in the prior art. These functional advantages include an avoidance of dislocation in response to normal walking movement without reliance upon a fixed hinged connection. Very effective prior art knee joint prostheses that incorporate certain of the structural features referred to herein are disclosed in U.S. Pat. No. 4,470,158 and U.S. Pat. No. 4,309,778.
A prior art knee joint that permits rotational movement of the bearing on the tibial component is shown in FIGS. 1-6, and is identified generally by the numeral 100. The prior art prosthesis 100 includes a femoral component 102 for mounting to the distal end 104 of the natural femur 106. The femoral component 102 includes an inferior articular bearing face having medial and lateral condyles 108 and 110 as shown in FIG. 2.
The prior art prosthesis 100 further includes a tibial component 112 that is mounted to the resected proximal end 114 of a tibia 116. The tibial component 112 includes an inferior surface having a generally conical projection 118 configured to be engaged in a cavity prepared in the resected distal end 114 of the tibia 116. The tibial component 112 further include a general planar superior bearing surface 120. The bearing surface 120 is formed with a generally conical central recess.
The prior art knee prosthesis 100 further includes a bearing 122 disposed between the femoral component 102 and the tibial component 112. The prior art bearing includes a superior articular bearing surface 124 configured for articular bearing engagement with the condyles 108 and 110 of the femoral component 102. The bearing 122 further includes an inferior surface for engagement with the tibial component. More particularly, the inferior surface of the prior art bearing 122 includes a central projection (not shown) that is rotatably engaged in the recess that extends into the superior surface 120 of the tibial component 112. The inferior surface of the bearing further includes a generally planar bearing surface 126 that is in rotational bearing engagement on the superior planar bearing surface 120 of the tibial component 112.
The components of a knee joint undergo complex movement relative to one another during walking and other activities. In particular, natural movement of the knee joint causes rotation of the tibia about a generally medial to lateral axis. Simultaneously, the natural movement of the leg will generate some pivotal movement between the tibia and femur about an axis extending generally along the load bearing direction. This pivoting movement will cause some rotation of the prior art bearing 122 on the femoral and tibial components 102 and 112. Rotation of the bearing 122 relative to the femoral component 102 causes the condyles 108 and 110 of the femoral component 102 to climb upwardly toward the lips at the anterior and posterior ends of the bearing 122. This creates the potential for a spinout dislocation. The spinout can be controlled by proper attention to maintenance of collateral ligament tension during implantation. However, the cruciate ligaments are the principle anterior-posterior and medial-lateral stabilizers of the knee. These cruciate ligaments often are removed as part of the surgery to implant the prosthetic knee. Thus, the potential for dislocation exists with prior art knee joint prostheses that employ a rotating bearing.
The instability of the prior art rotating bearing prosthetic knee joint is illustrated most clearly in FIGS. 1-6. In particular, the prior art rotating bearing 122 can be forced to rotate to a dislocated position under the action of combined effects of an anterior-posterior shearing load, distraction of one of the condylar components 108 and 110 of the femoral prosthetic component 102 and a lax collateral ligament associated with the distracted component. Only ligament tension sufficient to prevent the femoral condyle on the distracted side from climbing over the lip of the bearing can prevent such dislocation. There is no other sufficient soft or hard tissue in the knee to prevent such dislocation. The dislocation mode is stable under compressive load. Both anterior-posterior and medial-lateral shift of the prior art femoral component 102 relative to the tibial component 112, as illustrated in FIGS. 5 and 6, must accompany such dislocation. The shearing force and the effect of the vertical rotational axis of the bearing accentuate conditions that can lead to the illustrated spinout. Spinout does not occur in the prior art prosthetic joints where there is an allowance for anterior-posterior translation of the bearing. Thus, the illustrated spinout is a unique disadvantage of a fixed axis rotating platform tibial bearing knee prosthesis as illustrated in FIGS. 1-6.
One prior art technique for minimizing spinout dislocation involves the use of a bearing with increased engagement between the bearing and the femoral component. Such a bearing requires a greater degree of distraction to allow spinout. Although prosthetic knee joints of this type reduce spinout, they do not completely eliminate the occurrence of spinout.
The prior art also includes a rotational stop on the patellar component of the prosthetic knee joint marketed under the name New Jersey LCS. The rotational stop, however, has not prevented spinout of the rotating patella bearing.
In view of the above, it is an object of the subject invention to provide a successful anti-spinout stop joint that will not adversely affect the function of the prosthetic joint.
It is also an object of the subject invention to provide a prosthetic knee joint that can provide enough rotary motion for the needed functions, while still preventing spinout.